Chestwall compliance was determined in 26 premature infants (BW 1 320 +/- 410 g, gest. age 32 weeks) and in 10 full-term infants (BW 3 155 +/- 810 g) who were ventilated mechanically. Chestwall compliance in premature infants was 6.4 ml/(cmH2O X kg), decreasing with advancing gestational age to 4.2 ml/(cmH20 X kg) in full-term infants. There was a linear correlation (r = 0.95 and 0.79 respectively) between tidal volume and the pressure transmitted to the esophagus throughout the tidal volume range. The protion of airway pressure transmitted to the infants with hyaline membrane disease, 12% in newborns with a patent ductus arterisus, 17% in normal prematures and 25% in normal full-term infants. The findings suggest that during mechanical ventilation the high chestwall compliance and low lung compliance of premature infants prevent a significant rise in intrapleural pressure which could interfere with central venous return and cardiac output. However, using high inspiratory pressures and continuous distending airway pressure in the absence of lung pathology may result in a decreased cardiac output. The highly compliant chestwall of the premature infant may exert insufficient outward recoil and might be one of the causes of a low functional residual capacity and chronic pulmonary failure in the premature infant.